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When the Douglas DC-3 entered service with American Airlines, Adolf Hitler was in power in Germany, Jesse Owens was the fastest man in the world and the history of powered flight was barely three decades old.

Seventy-six years later, the two-engined airliner and cargo plane – with its distinctive rumbling sound – is still a regular sight in our skies. And not just for air shows or charter scenic flights; DC-3s, and their military version the C-47, are still serving as freighters and airliners.

Last May saw the longest running production for a military jet finally end – the Soviet Union’s MiG-21, an interceptor capable of flying at twice the speed of sound and whose delta shape became an icon of Cold War crises and superpower proxy wars. It first flew in 1959, and was later produced in China.

When so few aircraft outlast the lives of their designers, what qualities set old-timers like the DC-3 and the MiG-21 apart? Here, Royal Aeronautical Society Head of Research Keith Hayward and design writer Jonathan Glancey outline the five rules every classic should have.

Rule one: Be adaptable and flexibleLaunched in 1969, the Harrier jump jet typifies this rule, says Glancey. “It remains in service with the US Marine Corps – beginning its days as a last ditch Cold War interceptor based as close as possible to the East German border, and ending its RAF days as a sophisticated patrol-and-strike aircraft in Afghanistan.”

Adaptability and flexibility is also key with civilian aircraft. “Most modern civil aircraft are now designed with further developments in mind, as well as sharing features and parts to reduce training and operational costs,” says Hayward. “Airbus is probably more successful in this respect as a family than Boeing has been.”

Rule two: Be easy to fly"Aircraft that stand the test of time tend to be simple and reassuring machines to fly,” says Glancey. “This means reliable engines, smooth, precise controls and crystal-clear instruments that are easy to reach or see – especially when upside down, in combat, in unfamiliar conditions and emergencies. “

Hayward agrees. “[The] basic principle is that the design must fulfil basic objectives of its ‘market’,” he says. “Military aircraft fully meets mission requirements, with high chance of bringing crew back alive; an airliner makes money for airline, ditto passengers and crew safely delivered on a regular basis.”

Rule three: Be resilient“The B-52 – which entered service in 1955 and is expected to fly until at least 2040 – has and will outlast other bombers because it has solid basic design qualities, structural resilience, and simply no better bomb truck has turned up since,” Hayward says.

“Aircraft like the Lockheed F-104 Starfighter and the Convair B-58 Hustler supersonic bomber might have been fast, furious and downright thrilling to fly on a good day,” says Glancey, “yet both have accident rates to match: the Starfighter was known as the ‘Widowmaker’, while a fifth of all Hustlers were lost to accidents.”

Rule four – Be easy to maintain“If you can keep a veteran MG car on the road, you should be able to service and maintain a Tiger Moth, more or less,” says Glancey. “No one likes a cantankerous aircraft not least because its safety – the crew’s safety, your safety – turns on the machine behaving impeccably. As air passengers used to say in the States – ‘If it’s not Boeing, I’m not going.’ Boeing, along with other US manufacturers, made aircraft that were comfortable, reliable, safe and easy to maintain.”

“Outlasting the field may also explain why ex-Soviet designs like the MiG-21 or the Antonov An-2, a biplane first flown in 1947 and still in production in China, have stayed in service,” says Hayward. “All are cheap, simple, brick outhouse constructions, with little or no competition in their original markets.”

Rule five – Be easy on the eye“In its purest form, the Hawker Hunter is such a beautiful machine, that it must have been a wrench for RAF pilots to abandon it,” says Glancey. “Swiss pilots hung on to their Hunters until 1994, and the aircraft lives on in front-line service with Lebanese Air Force today. But, beauty and sentimentality alone cannot stop the march of new technology, even though I can’t help thinking they’ve played their part in keeping a number of aircraft in active service well past what might have seemed their natural retirement date.”

But don’t be swayed by looks alone, warns Hayward. “The F-4 Phantom was one of the ugliest beasts in the flying world, but it fought several wars, operated on land and at sea and outlasted many prettier aircraft,” he says. “Concorde was lovely and was an immensely impressive piece of technology, but it failed in its basic function as an airliner, to sell widely [to airlines] and to make money.”

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